Yeah, Exum was that guy last year, at least defensively. Just don't think Neto or Burke's offense was strong enough to make up for their poor defense. Mack is more well-rounded than those two, and he plays a very different style from Burke and Neto.
I'd still rather have Exum in this spot, as his defense is unquestionably the best of the four of them, and he looked good shooting the ball too, but Mack has been a revelation.